With many printing or image recording apparatus, such as laser printers, photocopiers, or facsimile machines, it is often desired to record the printed images of a document on both sides of each sheet of paper (duplex recording). So far, known methods and apparatus have not provided a sufficient solution to the problem of duplex recording.
In accordance with one known method and apparatus for duplex recording, the paper sheets are fed through an image-forming unit and a reversing unit. First, the paper sheets are fed through the image-forming unit where printed images are recorded on one side of each sheet. The sheets of paper are then reversed or flipped over by the reversing unit and fed again through the image-forming unit to record the printed images on the other sides of the sheets. For example, if a sixty page document is to be duplex recorded on thirty sheets of paper, the images are first recorded on one side of each of the thirty sheets of paper in the image-forming unit. While the first sides of the sheets are printed, the sheets are then fed one at a time through the reversing unit and stacked on top of one another in an intermediate paper tray. The stack of reversed sheets are then fed again one at a time through the image-forming unit to record the printed images on the other side of each sheet. This method of duplex recording is often referred to as the stack method, because after the sheets of paper are printed on one side, all of the sheets of the document are stacked on top of one another in an intermediate paper tray, prior to recording the printed images on the other sides of the sheets.
One drawback of the stack method of duplex recording is that when the sheets of paper are temporarily stacked on top of one another in the intermediate paper tray, the sheets then have to be separated from one another and discharged one at a time into the image-forming unit. This paper separating procedure can be particularly troublesome because whenever two or more sheets stick together they are usually then fed through the image-forming unit on top of one another. When this occurs, the pagination of the document becomes incorrect and, if the machine does not catch the error, the document, or at least a portion of it usually has to be reprinted.
Another drawback of the stack method of duplex recording is that different sized paper sheets usually cannot be stacked together in the intermediate paper tray. The intermediate paper tray usually has a pair of guide plates mounted on either side thereof, that are laterally movable relative to the stack to align the stack in the tray, so that the paper sheets can be properly refed through the image-forming unit. If one group of sheets in the stack is wider than another group of sheets, the guide plates will only come into contact with and, therefore, will only align the wider group of sheets. As a result, the narrower paper sheets will likely not be properly aligned when fed through the image-forming unit and thus the printed images will not be properly recorded thereon.
Another problem with the stack method of duplex recording, particularly if duplex recording with a photocopier or facsimile machine, is that if one or more sheets of paper become jammed in a paper feed path, it is difficult to determine which sheet or sheets were jammed and which have to be reprinted. Therefore, when such a jam occurs, frequently, either the entire document has to be reprinted or the operator has to undergo the tedious process of looking through the entire stack of printed sheets to determine which sheets were jammed in order to reprint both sides of those sheets.
Yet another drawback of the stack method of duplex recording and, in particular, when transferring electronically stored information to a hard copy, is that with larger documents, a large amount of computer memory is required to store the printed image information for the second sides of the sheets to be recorded. For example, if a fifty sheet double-sided document (100 pages) is to be printed, all 100 pages of the document are first computed and stored in computer memory. However, if the intermediate paper tray cannot hold all fifty sheets, then the pages for the maximum number of sheets that the intermediate paper tray can hold are computed and stored in computer memory. Then, the fifty odd-numbered sides of the sheets are consecutively printed and the sheets are stacked in the intermediate paper tray. The sheets are then fed again through the image-forming unit, starting with the top sheet on the stack, or, that is, the last sheet to be printed on its odd-numbered side, and the printed image information is then recalled from memory for printing the images on the even-numbered sides of the sheets. Therefore, for larger documents, a large amount of computer memory is required to first store the printed image information for the entire document, or the maximum number of sheets that the intermediate paper tray can hold. The printed image information for 100 pages, for example, is extremely large (one sheet of 81/2.times.11 inch paper requires about 8 million bits when imaged at 300 dots per inch). Because it is generally necessary to use the computer memory as efficiently as possible, its use for storing a relatively large amount of printed image information frequently compromises the overall performance and capabilities of the computer system. Likewise, to add additional memory to either the computer or printing apparatus for storing the printed image information for larger documents is often prohibitively expensive.
Another problem of the stack method of duplex recording is that it can be relatively time consuming. For example, in duplex printing a 100 page document, as described above, first the entire 100 pages have to be computed and stored in computer memory before the printing begins. Then, the 50 odd-numbered pages are printed and the sheets are stacked in the intermediate paper tray. Before the even-numbered pages can be printed, the duplexing feed path must be emptied and the last odd-numbered page has to be printed and placed on top of the stack. The last odd-numbered sheet is then fed from the top of the stack as the first even-numbered sheet to be printed, and the even-numbered pages are then printed on the entire stack. The steps of first computing all of the pages of the document, and emptying the duplexing feed path between the odd-numbered page printing mode and even-numbered page printing mode are relatively time consuming and, therefore slow down the overall rate of duplex printing the document.
Another problem with known image-recording apparatus is that frequently the control system of the apparatus cannot distinguish between most emergency and non-emergency stop conditions. A non-emergency stop condition might be, for example, a low toner supply or a low paper supply. Whereas an emergency stop condition is usually a problem that could cause a printing malfunction or error, such as a paper jam in a feed path. Therefore, even when there is a non-emergency condition, some known apparatus will immediately stop in order to treat the problem. As a result, if the apparatus is stopped for a long enough period of time to treat the condition, the paper stopped in the feed path may become curled depending on the shape of the feed path, the type of paper, and the temperature and/or humidity conditions. When the apparatus is started again, the curled sheet of paper will then frequently become jammed in the feed path.
It is an object of the present invention, therefore, to provide an improved method and apparatus for duplex recording that overcomes the problems, drawbacks and disadvantages of known methods and apparatus.